Prior to present invention several processes had been disclosed in the literature for the preparation of thiobiscarbamates. For example U.S. Pat. No. 4,004,031 which issued on Jan. 18, 1977 to Jozef Drabek, disclosed the preparation of bis-]O-(1-alkylthioethylimino)-N-methylcarbamic acid]-N,N'-sulfides which were indicated to be useful as insecticides. However, it has been subsequently observed that this method results in relatively low yields of the desired sulfide and high yields of undesired by-products. Moreover, it has also been observed that the product is not sufficiently stable and hence coupled with low yields it is unattractive for large scale production as a commercial pesticide.
In U.S. Pat. No. 4,256,655 which issued on March 17, 1981 to R. W. Ashworth and W. Y. Fu there is also disclosed a method for making thiobiscarbamates which involves the preparation of a nitrogen-containing heterocyclic base with a sulfur chloride and thereafter, in the presence of a solvent reacting this pre-formed adduct with a carbamate. While this method represented a substantial improvement in yield of the desired thiobiscarbamate, over earlier methods, the stability of the product for commercial use was still not reliable.
In applicant's copending application Ser. No. 199,382 filed on Oct. 21, 1980 now U.S. Pat. No. 4,323,516, , a further improvement to the method for making thiobiscarbamates is provided. The method disclosed therein also involves the pre-formation of an adduct of solvent and sulfur chloride and thereafter reacting the adduct with the carbamate and washing the reaction product with water or alcohol.
Prior to the present invention, however, none of these methods gave a product in sufficiently high yield and of acceptable stability to be commercially attractive. Accordingly, one or more of the following objects can be achieved by the practice of this invention. It is an object of this invention to provide an improved process for the preparation of thiobiscarbamates which exhibit excellent stability characteristics. Another object of this invention is to provide an improved process for the preparation of thiobiscarbamates in high yields. A further object of this invention is to provide a process which involves the in situ reaction of a carbamate with sulfur dichloride in the presence of a nitrogen-containing heterocyclic base. A still further object is to provide a process for the preparation of thiobiscarbamates wherein the reaction is quenched with methanol at low temperatures. Another object of this invention is to provide an improved process for the preparation of bis-[O-(1-methylthioethylimino)-N-methylcarbamic acid]-N,N'-sulfide in high yields and which is sufficiently stable for commercial applications. These and other objects will readily become apparent to those skilled in the art in light of the teachings hereinsetforth.